Cubic boron nitride is second to diamond in hardness and has higher chemical stability than diamond. Thus, cubic boron nitride is increasingly employed as grinding material, polishing material and cutting material. A variety of methods for producing cubic boron nitride have been proposed. Among them, best known and most widely employed in the industrial field is a method in which hexagonal boron nitride is maintained in the presence of a catalyst substance under conditions where cubic boron nitride remains thermodynamically stable (approximately 4.0 to 6.0 Gpa and approximately 1400 to 1600° C.) to thereby transform hexagonal boron nitride into cubic boron nitride. U.S. Pat. No. 3,772,428 and JP-B SHO 61-283, JP-B HEI 5-94 and JP-B HEI 5-95 disclose, as a catalyst substance, alkali metals, alkaline earth metals, nitrides thereof and boronitrides thereof. More specifically, U.S. Pat. No. 3,772,428 discloses Li3N and Li3BN2 as particularly promising catalyst substances. However, cubic boron nitride produced by use of any one of these catalyst substances generally assumes the form of microparticles having a size of 50 μm or less and contains insufficiently grown crystal planes. Thus, abrasives produced from such cubic boron nitride have not yet attained satisfactory performance.
JP-B SHO 61-283 discloses LiCaBN2 as a promising catalyst substance. Cubic boron nitride produced by use of the catalyst substance generally has a quasi-spherical shape and exhibits excellent mechanical strength. JP-B HEI 5-94 and JP-B HEI 5-95 disclose a method in which a mixture of LiMBN2 (M represents an alkaline earth metal) with Li8SiN4 or Ca5Si2N6 is used as a catalyst substance. Cubic boron nitride obtained through the method has a well-grown (111) crystal plane and exhibits excellent mechanical strength.
However, these methods have not yet attained satisfactory percent transformation of hexagonal boron nitride into cubic boron nitride. Thus, a catalyst substance that attains high percent transformation is desired in industrial process. In addition, higher mechanical strength of the catalyst substance is also desired.
An object of the present invention is to solve the problems residing in the aforementioned conventional techniques and to provide cubic boron nitride improved in mechanical strength and a grindstone having enhanced grinding performance.
Another object of the invention is to provide a method for producing cubic boron nitride that enhances percent transformation of hexagonal boron nitride to cubic boron nitride.